Experimental investigation of nitrogenation process for heavy rare earth nitrides from their hydrides

We experimentally investigated the nitrogenation process of heavy rare earth (Gd, Tb, Dy, Ho and Er) hydrides dynamically by thermal analysis. The onset temperature of nitrogenation and the absolute value of nitrogenation enthalpy increased as the atomic number of the heavy rare earth elements increased and the onset temperature of nitrogenation with a sharp exothermic peak exhibited a large gap between Tb (406 °C) and Dy (806 °C). The reaction route R→RHx→RN has been done at a relatively low temperature and atmospheric pressure, indicating that the diffusion coefficient of nitrogen into the rare earth hydride is much higher than that of nitrogen into the rare earth metal and/or the activation energy of RHx→RN is lower than that of R→RN. For complete nitriding, ErN requires the highest nitrogenation temperature of at most 1000 °C in N2 flow from ErHx. We obtained the magnetic entropy change ΔS of -14 J/kg K (144 kJ/m3 K, calculated by using the theoretical density of 10.3 g/cm3) at ΔH = 2 T for HoN prepared at the nitrogenation temperature of 1000 °C. This value reached the reported value, indicates that this simple synthesis route without any toxic compounds as a precursor and with normal equipment of an atmospheric furnace without high temperature and pressure can adequately exploit the ability of the rare earth nitride as a magnetic refrigerant for magnetic refrigeration.